We hypothesize that a donor dendritic cell (DC) population in the rhesus macaque can be identified and purified to allow its effective employment in tolerance-enhancing strategies in renal transplantation. Using technologic approaches that we have applied successfully in humans to identify circulating (P)DCl and pDC2, we have generated preliminary data that demonstrate presumptive counterparts of these DC precursors/immature DC in the rhesus macaque. In AIM I, we shall further characterize these cells to validate their relationship to human (and mouse) DC subsets, and to establish their influence on allogeneic T cell responses, particularly in relation to their postulated tolerogenic effects and underlying mechanisms. Use of hematopoietic growth factors (Flt3L and G-CSF) will greatly assist in addressing this Aim, Since. in vivo generated immature myeloid (M)DC presently represent the "standard" tolerogenic DC, we shall compare and contrast the influence of in vivo-mobilized pDCl and pDC2 with that of their immature, in vitro-generated MDC counterparts. Based on the outcome of AIM I, we shall in AIM II, test the most promising putative tolerogenic rhesus DC subset in allogeneic recipients using established and cutting edge assays of anti- donor reactivity .We shall also investigate underlying mechanisms, in particular whether infusion of putative tolerogenic DC results in immune deviation. Although other non-exclusive mechanisms may be operative, we shall focus on the ability of rhesus DC to skew towardsTh2 predominance as immature murine DC and human pDC2 have been shown to promote this mechanism. In AIM III, we shall work closely with our collaborators at Emory University Transplant Center and the Yerkes Primate Research Center in Atlanta. We shall test the donor DC population shown to exhibit the maximal tolerogenic potential in AIM II, in an established; MHC-mismatched rhesus renal transplantation model. Transplantation will be performed in conjunction with anti-CD40L mAb treatment of the recipient that, we hypothesize will markedly potentiate the tolerogenic effect of immature donor DC. Since deoxyspergualin has been shown recently to prevent DC maturation in rhesus renal allograft recipients in a tolerance- promoting regimen, we shall retain the option of incorporating this additional agent to further potentiate the tolerogenic action of immature donor DC combined with costimulation blockade.